Soluble mediators of inflammation such as the complement derived anaphylatoxins play important roles in a variety of immunologically mediated human systemic and cutaneous diseases. We have purified human C5a and C5a des Arg and studied their in vivo and in vitro reactivity. Their in vivo role was assessed by the first in-depth analyses of the cutaneous reactivity of these complement fragments in man. We have also documented the ability of C5a and C3a to modulate cell surface receptors for immunoglobulin and complement on the surface of leukocytes. We have assessed the contribution of polymorphonuclear neutrophils (PMN's) and mast cells to C5a induced cutaneous inflammation by studying the cutaneous reactivity of patients with bone marrow failure who lack PMN's and by selectively depleting the mast cells in the skin of volunteers prior to skin testing with C5a. Increasing evidence indicates that human endothelial cells, under certain circumstances, can be induced to become immunologically competent. We have isolated human umbilical vein and dermal microvascular endothelial cells grown them in cell culture, examined then for the presence of immunologically relevant cell surface antigens and receptors before and after stimulation with soluble mediators of immunoregulation. In addition we have induced human endothelial cells to differentiate in vitro. Under specific culture conditions the endothelial cells undergo angiogenesis forming small tubular structures that possess lumens and resemble blood vessels. We have also developed an assay to detect antiendothelial cell antibodies in huma sera and have detected these antibodies in patients with certain forms of vasculitis.